In the rotary drilling of oil wells, drilling fluid is introduced to the drilling area through the drill string for several reasons. It cools and lubricates the drill bit at the bottom of the drill string and carries the cuttings to the surface through the annular space between the drill pipe and the wall of the well bore. Fluid in the annulus also provides a static head which assists in maintaining the hydrostatic pressure in the well bore greater than the formation pressure, thereby preventing the intrusion of gas or liquid from a subterranean zone containing formation fluids.
Although drilling fluid provides these beneficial results, it can also be the cause of potential problems. If the difference between the hydrostatic pressure and the formation pressure is great enough, drilling fluid will tend to flow into intersected permeable formations. It is not only expensive to lose this drilling fluid, but if the amount of fluid flowing into the formation results in the loss of circulation of the drilling fluid, the drill bit may bind and the well bore may collapse. Moreover, drilling fluids lost in the formation can plug the formation, eventually resulting in less production from the well.
One way of preventing or controlling the loss of drilling fluid is to aerate the fluid. It has been recognized that by reducing the density of the fluid the hydrostatic pressure exerted by the column of drilling fluid in the well bore is also reduced. By controlling the hydrostatic pressure the amount of fluid lost in permeable formations can be minimized, making the process less costly and making it less likely that the fluid will plug adjacent formations or that fluid circulation will be lost. Typically, the amount of air to be included in the drilling fluid in any particular drilling operation is determined, and air compressors are provided to deliver that amount into either the drilling fluid discharge line or the drilling fluid return line. In either case, injection of air into the drilling fluid at a fixed rate can result in fluctuation of the hydrostatic pressure of the fluid in the well bore as operating conditions change. For example, fluctuations in pump speed, drilling fluid volumes, injection pressures and weight on the drill bit can cause pressure variances downhole, resulting in variances in the ratio of air to drilling fluid and consequent variances in density and hydrostatic pressure.
In order to overcome these problems, it is an object of the invention to provide an aeration system which is able to control the flow of compressed air into the drill string so as to automatically maintain a constant hydrostatic pressure in the well bore.